Olfactory Nerve (CN I): Anatomy, Function, and Sensory Health
Updated: 6 days ago
Medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: October 17, 2026
The olfactory nerve, or cranial nerve I (CN I), is the nerve behind your sense of smell. It is the shortest sensory nerve in the body, and it is more important than it sounds: smell shapes flavor, appetite, and your ability to detect danger. This guide explains how it works, what can damage it, and how to protect it.
Quick Answer: What is the olfactory nerve?
The olfactory nerve (CN I) is the first pair of cranial nerves and the shortest sensory nerve, carrying signals from smell receptors in the upper nose to the brain. Damage can cause reduced, lost, distorted or phantom smells, from causes such as sinus infections, head injury, toxins and COVID-19. See a provider if your sense of smell changes suddenly.
What the research says
Moderate to severe head injury is strongly linked to smell loss. A systematic review of 19 studies (951 patients), with 11 studies in the meta-analysis, found a large effect for olfactory dysfunction in moderate-to-severe traumatic brain injury compared with healthy controls (Hedges' g = −2.43, 95% CI −3.16 to −1.69). The effect was larger with older age and with a higher share of women in the patient group (Tai et al., Neuropsychol Rev, 2023) [2].
Olfactory training helped people with long-lasting COVID-19 smell loss. A meta-analysis of 9 studies (179 patients in 7 studies with Sniffin' Sticks data) of training lasting more than 8 weeks for olfactory dysfunction lasting more than 16 weeks found significant improvements in the combined TDI score and in threshold, discrimination, and identification tests (Chen et al., Rhinology, 2025) [3].
Study details retrieved from PubMed.
What is the anatomy and sensory pathway of the olfactory nerve?
The olfactory nerve originates directly from the cerebrum, the upper part of the brain, and is the shortest sensory nerve in your body. Its fibers pass through the cribriform plate, a spongy, lightweight skull bone that separates the nasal area from the brain, and then reach the olfactory bulb, which sits just above the cribriform plate inside the skull, under the front of the brain. [1]
The smell-sensing lining of the upper nose is called the olfactory mucosa. It contains three main cell types: olfactory receptor cells, whose dendritic processes reach tiny hairs in the mucosa and whose central processes send signals toward the brain; sustentacular cells, which support nearby tissue; and basal cells, which develop into new receptor and sustentacular cells. [1]
Component | Location | Role |
Olfactory mucosa | Upper part of the nasal cavity | Contains receptor cells that detect smell molecules |
Cribriform plate | Skull bone between nose and brain | Fibers pass through it on the way to the bulb |
Olfactory bulb | Inside the skull, just above the cribriform plate | Receives smell signals from the nerve fibers |
For a broader look at the nose, see Understanding the Nose: More Than Just Your Sense of Smell, and for the full set of cranial nerves, Understanding Your Cranial Nerves.
How does the olfactory nerve make smelling work?
You smell in two ways. In orthonasal smelling, molecules are inhaled through the nostrils. In retronasal smelling, chewing or sipping releases molecules that travel up from the back of the throat to receptors in the back of the nose. [1] Retronasal smell is a major part of how you perceive the flavor of food and drink.
Smelling Method | Path of Molecules | Main Role |
Orthonasal | Inhaled through the nostrils | Detecting smells and hazards in the environment |
Retronasal | From the throat up to the back of the nose while eating or drinking | Perceiving the flavor of food and drink |
Which conditions and symptoms affect the olfactory nerve?
Olfactory nerve problems that affect smell can be caused by sinus infections, nasal polyps, tobacco use, head injuries, some medications, COVID-19 and diseases such as Parkinson's disease.
Term | What It Means |
Anosmia | Complete loss of the sense of smell |
Hyposmia | Partial loss of the sense of smell |
Parosmia | Distorted smell; familiar items such as food may smell like chemicals or mold |
Phantosmia (dysosmia) | Unpleasant or strange odors that occur spontaneously, with no source |
Possible causes. Cleveland Clinic lists sinus infections and nasal polyps, tobacco use, poor dental hygiene, environmental toxins such as insecticides, severe head injuries including concussions, some medications such as antibiotics, SARS-CoV-2 (COVID-19), head and neck cancer, diabetes, Alzheimer's disease, brain tumors, Parkinson's disease, and epilepsy. [1]
Related guides: Anosmia (Loss of Sense of Smell), Chronic Sinusitis, Nasal Polyps, and Post-Concussion Syndrome.
COVID-19 and Smell
Up to half of people with COVID-19 experience loss of smell. Researchers are still exploring why, but the virus may damage the smell receptors. Many people regain their sense of smell, but it can take several months. [1] See our guides to COVID-19 and Long COVID.
Why does olfactory nerve smell loss matter for safety?
Losing your sense of smell can lead you to eat too much or too little, because food does not taste the same. It can also make you miss danger signs such as a gas leak, smoke, or spoiled food. [1] If your sense of smell is reduced, make sure your home has working smoke and carbon monoxide detectors, and check expiry dates on food rather than relying on smell.
When should you see a healthcare provider about olfactory nerve problems?
Contact a healthcare provider if you notice a change in how things smell, a sudden loss of smell, or a foul odor with no apparent cause. [1]
How can you protect your sense of smell and olfactory nerve?
Tip | Why It Helps |
Prevent head injuries | Wear helmets during sports and cycling; avoid dangerous activities that could cause brain injury |
Protect your nose from chemicals | Wear a mask around insecticides and other chemical fumes |
Stay current on vaccines | Cleveland Clinic includes getting the COVID-19 vaccine among its prevention tips |
Quit tobacco | Quit smoking, vaping, and other tobacco use; see our guide to nicotine replacement therapy |
Keep up dental hygiene | Brush and floss regularly; poor dental hygiene is listed among the causes of smell problems |
Treat related conditions | Stay current with treatments for conditions that affect smell |
What is the bottom line on the olfactory nerve?
The olfactory nerve is a short but important link between your nose and your brain. It underlies not just the pleasure of scent and flavor but also your safety and nutrition. Head injuries, infections, toxins, tobacco, and some diseases can damage it, and COVID-19 has made smell loss more familiar. Protect it with helmets, masks around chemicals, no tobacco, and good dental hygiene, and talk to a healthcare provider if your sense of smell changes suddenly.
Frequently Asked Questions
What is the olfactory nerve?
The olfactory nerve is the first pair of cranial nerves (CN I) and the shortest sensory nerve in your body. It carries smell signals between the nose and the brain.
Where does the olfactory nerve start and end?
Per Cleveland Clinic, it originates directly from the cerebrum, the upper part of the brain, and its fibers pass through the cribriform plate to reach the olfactory bulb, which sits inside the skull just above the plate.
What is the difference between anosmia and hyposmia?
Anosmia is the complete loss of the sense of smell. Hyposmia is a partial loss.
What is parosmia?
Parosmia is a distorted sense of smell, in which familiar things, such as certain foods, may smell like chemicals or mold.
How does COVID-19 affect the sense of smell?
Up to half of people with COVID-19 experience loss of smell. Researchers are still exploring why, and the virus may damage olfactory receptors. Many people regain their sense of smell, but it can take several months.
Can head injuries cause smell loss?
Yes. Severe head injuries, including concussions, can damage the olfactory system. A meta-analysis found prominent smell loss after moderate to severe traumatic brain injury.
What is the cribriform plate?
The cribriform plate is a spongy, lightweight skull bone that separates the nasal area from the brain. Olfactory nerve fibers pass through it.
Does the olfactory nerve affect how food tastes?
Yes. When you chew or sip, molecules travel up from the back of the throat to receptors in the back of the nose (retronasal smell). This is a big part of how you perceive flavor.
What is phantosmia?
Phantosmia, also called dysosmia, is smelling unpleasant or strange odors that occur spontaneously and have no source.
Why is losing your sense of smell a problem?
Losing your sense of smell can lead to eating too much or too little, and it can make you miss danger signs such as a gas leak, smoke, or spoiled food.
Related Resources
References
Cleveland Clinic. Olfactory nerve. Cleveland Clinic. Medically reviewed; last updated May 21, 2022. https://my.clevelandclinic.org/health/body/23081-olfactory-nerve
Tai K, Leland EM, Seal SM, Schneider ALC, Rowan NR, Kamath V. Olfactory dysfunction following moderate to severe traumatic brain injury: a systematic review and meta-analysis. Neuropsychol Rev. 2023;33(4):717-732. PMID: 36070126. DOI: 10.1007/s11065-022-09563-2
Chen CH, Shih CF, Hummel T, Chao YT. Multidimensional benefits of olfactory training for chronic COVID-19-related olfactory dysfunction: a systematic review and meta-analysis. Rhinology. 2025;63(4):431-440. PMID: 40465208. DOI: 10.4193/Rhin24.355
Medical disclaimer
This article is for general informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional about your health.
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